The origin of modern humans (Homo sapiens) is one of the most controversial problems in archaeology and anthropology.

In order to understand the evolution of modern humans first we have to look at the anatomical and physiological modifications. Anatomically speaking, the evolutionary shift from some kind of Homo erectus ancestor to Homo sapiens involved the decrease of skeletal and dental robusticity, modifications of certain functional particularly locomotor anatomy, and an increase in cranial volume. Behaviorally, the transition brought with it a more finely crafted tool technology, more efficient foraging strategies and artistic expression. (Lewin, 2005, p. 149) This has been particularly true since 1986 and the publication of the first mitochondrial DNA studies suggesting that modern humans may have had their origin in Africa. Current opinion suggests that there may have been multiple dispersals of modern humans out of African during the past 200,000 years or so. The discovery of modern humans in 2002 at the site of Herto in Ethiopia provides convincing proof that modern humans were living in Africa 160,000 years ago, at a time that Neanderthals were in Europe and Homo erectus may still have survived in Java. (Renfrew and Bahn, 2005, p. 55) The transition for Middle Paleolithic to the Upper Paleolithic at around 50.000 years ago coincides with the expansion of Homo sapiens and the demise of other hominin species. (Pettitt, 2005, p.125)

Out of Africa model. About 100.000 years ago, the African stock of modern humans started to spread from the continent into adjoining regions and eventually reached Australia, Europe and the Americas. (Stringer and Andrews, 2005, p. 142) Like the multiregional model, this view accepts that fossils assigned to Homo erectus evolved into new forms of humans in inhabited regions outside Africa, but argues that these non-African lines become extinct without evolving into modern humans. (Stringer and Andrews, 2005, p. 143) [Modern humans] are certainly coming out of Africa, but were finding evidence of low levels of admixture wherever you look says evolutionary geneticist Michael Hammer of the University of Arizona in Tuscon. (Gibbons, 2011, p. 392) The broad line of evolution is pretty clear: Our ancestors came out of Africa says biological anthropologist John Relethford of the State University of New York College at Oneonta. (Gibbons, 2011, p. 393) Beginning in the late 1960s, however, a number of scholars, including Louis Leakey, W. W. Howells and, later, Chris Stringer and Peter Andrews, suggested that modern human origins could be traced to a single geographic centre, most often identified as Africa. Howells referred to this as the Noahs Ark hypothesis, and more recent terms have included the Garden of Eden, single origins and Out of Africa hypothesis. (Pettitt, 2005, p. 127)1

Stringer and others first proposed Africa as the birthplace of modern humans back in the mid-1980s. The same year, researchers published a landmark study that traced the maternally inherited mitochondrial DNA (mtDNA) of all living people to a female ancestor that lived in Africa about 200.000 years ago, dubbed mitochondrial Eve. (Gibbons, 2011, p. 393) [] the mitochondrial Eve hypothesis was consistent with the recent single-origin (Out of Africa) model and gave no support for the multiregional evolution model. (Lewin and Foley, 2004, p. 401) The Mitochondrial Eve hypothesis is the genetic equivalent of the Out of Africa hypothesis, which is based on fossil evidence. (Lewin, 2005, p. 155)

The second classic hypothesis is the multi-regional evolution model, which supports the idea that after Homo erectus left Africa and dispersed into other portions of the Old World, regional populations slowly evolved into modern humans. all living humans derive from the species Homo erectus that left Africa nearly two million-years-ago natural selection in regional populations, ever since their original dispersal, is responsible for the regional variants (sometimes called races) we see today the emergence of Homo sapiens was not restricted to any one area, but was a phenomenon that occurred throughout the entire geographic range where humans lived According to this view, when Homo erectus dispersed around the Old World over a million years ago, it gradually began to develop both the modern features and the regional differences that lie at the root of modern racial variation. Particular features in a given region developed early on, and persist in the local descendant populations of today. (Stringer, 2005, p. 140) Milford Wolpoff and Alan Thorne have emphasizes the importance of gene flow (interbreeding) between the regional lines, which prevented them from diverging and speciating, and allowed new traits to spread from one population to another across the inhabited world. (Stringer, 2005, p. 140) In the 1940s, Franz Wedenreich (1947) suggested that modern human had a multi-regional origin across the Old World. Regional continuity (Pettitt, 2005, p. 127) Central to the multi-regional argument are the fossils from Asia and Australia, which have been interpreted as showing evidence for regional continuity in populations. (Pettitt, 2005, p. 128) Fossils from Java, China and Australia play a central role in this debate multi-regional h. Pettitt, p. 1302

There are other models of recent human evolution apart from the extremes of Multiregional and Out of Africa. (Stringer and Andrews, 2005, p. 143) Relethford: My interpretation of the genetic fossil evidence is that our ancestry over the past several hundred thousand years is mostly, but not exclusively Out of Africa.

In 1928 Gunter Bruer proposed an Afro-European sapiens hypothesis that envisaged a degree of evolution in both regions with slow expansions out of Africa of emerging modern humans, and varying degrees of genetic mixing. He later described this as a hybridization and replacement model (Bruer 1989), which did not exclude a degree of regional continuity. [...] In 1989 Fred Smith and colleagues proposed an assimilation hypothesis, in which an emergent Homo sapiens population from Africa would have affected evolutionary processes in other regions, ultimately assimilating regional early human groups into the modern human gene pool (Smith, Simek and Harrill 1989). Furthermore, Marta Mirazn Lahr and Robert Foley (1994) suggested that multiple dispersals out of Africa were more probable than a single event, a notion that has been supported by genetic work (Templeton 2002). (Pettitt, 2005, pp. 128-129)

The American palaeoanthropologist Fred Smith proposed the Assimilation model, which depicts a more gradual spread of modern human features from Africa accompanied by intermixture with local human lineages in Europe and Asia. (Stringer and Andrews, 2005, p. 143) Assimilation model: the initial evolutionary changes leading to modern humans took place in Africa and then spread throughout the remainder of the species through the process of gene flow between neighboring populations. (Relethford, c2001, p. 65) Afro-European hybridization model (proposed by Gunter Bruer): the initial evolutionary changes leading to modern humans took place in Africa and then spread to populations outside of Africa. (Relethford, c2001, p. 65) Relethford: I would argue that both of these models are multiregional.

[] ancient DNA argued against the idea of mixing between Neanderthals and moderns. Over the years the replacement model became the leading theory. (Gibbons, 2011, p. 393) Population geneticists warned that complete replacement was unlikely, given the distribution of alleles in living humans. (Gibbons, 2011, p. 393)

Smith suggested that most of our ancestors arose in Africa but interbred with local population as they spread out around the globe, with archaic people contributing to about 10% of living peoples genome. (Gibbons, 2011, p. 393) Among the proponents of the recent replacement model there is a multitude of views about the exact rules, tempo and the number of Late Pleistocene migrations originating from Africa. (Mellars, 2007, p. 22) Forms of replacement: violence or Another possibility might be disease, brought in by newcomers and wrought on other humans that had no genetic resistance or prior immune experience. (Relethford, c2001, p. 56) Replacement can e perhaps best explained by natural selection operating through the competition of two separate species. (Relethford, c2001, p. 56)

Leslie Aiello (1993) distinguished clearly between the four main hypotheses: 1) an African replacement hypothesis, which argues that modern humans arose in Africa, dispersed from there, and replaced existing Homo species elsewhere, with little or no hybridization between the groups; 2) an African hybridization and replacement hypothesis similar to the former, but in which hybridization is variable but more significant; 3) an assimilation hypothesis, in which gene flow, admixture, and the effects of the already existing population on an incoming African population are significant; and 4) a multi-regional evolution hypothesis, which denies the primacy of Africa in the origin of modern humans and instead emphasizes long-term population continuity and gene flow. (Pettitt, 2005, p. 129)

My opinion: [] the Out of Africa model is more strongly supported than the multiregional evolution model. (Lewin and Foley, 2004, p. 440)

Bibliography Gibbons, A., 2011. A new view of the birth of Homo sapiens. News Focus, [online] Available at: www.sciencemag.org/content/331/6016/392.full.pdf [Accessed 07 November 2012]